Microsoft Windows operating system relies heavily on an internal database called windows registry, its used to record the many information details related to any installed program, hardware and other items in OS. However because of the frequent updates to this database, sometimes it gets corrupted due to many reasons like:
•    Installing Poor Applications
•    Spyware/Adware/Viruses that come from internet and other places.
•    Improper shutdown or power failure.
•    Installing many programs on one machine.
•    Unknown/Internal Bugs (rare)

These problems can make your computer slow, or sometimes prevents you from running certain tool/task correctly. Most viruses and spywares use the Run/RunOnce category in the registry to automatically launch themselves when you start the infected computer. Some other malware programs will modify some records that will make it hard if not impossible for average user to restore/fix the problem like preventing access to the task manager and system restore tools.

The simple answer to the question is that they want to sell you something from the internet or get access to your credit card/other account information. Many viruses, spyware, and malware programs force you to go and buy a fix/repair tools. Some will put links all over the desktop that will take you to the sales page of the fix tool. This illegal way of making money is highly spread and some people get their PC’s infected with a single click on a Google results link.

To find more about how to avoid and how to fix registry problems you can visit my site fixwindowsproblemsnow.com